Quoted from too-many-pins:

Bottom line when selling is you will get your best return selling each one at a time but this takes time, energy, etc. Selling in groups or "bulk deal" you will likely have to discount 20% (or so) but some people don't have the time or interest in selling one at a time.
Doing legwork is really the only way to get a great idea of value but again that takes time. Suggestions here - price guides - Boston Pinball average - etc will all help but depending on what your are looking to do and how quick you want to sell these selling one at a time will surely result in your best return